1. Far Field Signals
Unipolar or bipolar cardiac pacemaker lead systems fulfill two functions. The first function is to provide an electrical conduit by which a pacemaker output pulse is delivered to stimulate the local tissue adjacent to the distal tip of the lead. The second function is to sense local, intrinsic cardiac electrical activity that takes place adjacent to the distal end of the lead.
One of the problems with body implantable pacing and sensing lead systems is their inability to suppress or attenuate the voltage levels of far field electrical signals. These signals are generated by depolarizations of body tissues in areas remote from the local sensing site and are manifested as propagated voltage potential wavefronts carried to and incident upon the local sensing site. For example, a far field signal may comprise the intrinsic signal originating from the chamber of the heart opposite the one in which the lead electrode is located. Thus, where the lead electrode is implanted in the atrium, the ventricular QRS-wave comprises a far field signal; in contrast, for a ventricular implanted electrode, the atrial P-wave is the far field signal. The sensing electrodes detect or sense the voltages of these signals and interpret them as depolarization events taking place in the local tissue when such polarizations are above the threshold sensing voltage of the system. When far field signal voltages surpassing the threshold voltage are applied to the sensing circuitry of the pulse generator or pacemaker, activation of certain pacing schemes or therapies can be erroneously triggered.
With the development of universal stimulation/sensing systems, that is, three and four chamber combination pacemaker/cardioverter/defibrillators, accurate sensing of cardiac signals has become even more critical, and management, suppression, and/or elimination of far field signals is vitally important to allow appropriate device algorithms to function without being confused by the undesirable far field signals. As noted, an error in sensing can result in either a wrongfully delivered therapy or a wrongfully withheld therapy.
Approaches to the problem of far field signal sensing include configuring the circuitry of the pacemaker to attenuate far field signals, and introducing a blanking period long enough to prevent the sensing of unwanted signals. These solutions are described in U.S. Pat. No. 4,513,752 assigned to the owner of the present invention.
2. Net Signal Amplitude between Sensing Electrodes
U.S. Patent No. 5,306,292 teaches the use of multiple small electrodes on a lead tip for pacing and/or sensing. Each electrode has its own dedicated conductor and pacemaker connector terminal. The '292 patent discloses a scheme for selecting the best combination of electrodes for pacing and/or sensing. However, if two electrodes are selected for sensing a problem arises: For any two electrodes selected an orthogonal wavefront impinging on the two electrodes would result in a null output signal, that is, a net sensed signal having an amplitude of virtually zero volts which therefore would not be sensed by the device's circuitry.
U.S. Pat. No. 6,064,905 discloses a multi-element temporary mapping catheter including a plurality of small electrodes disposed about a tip section. As in the '292 patent, the electrodes in the '905 patent are each connected to a separate conductor. Accordingly, as in the '292 patent, a depolarization wavefront orthogonally incident on any pair of electrodes can result in a substantially zero net voltage signal. This is true also of the sensing electrode arrangement disclosed in U.S. Pat. No. 4,365,639 in which the electrodes are carried about the side surface of the lead body.
3. Ratio of Anode-to-Cathode Surface Areas
As illustrated by U.S. Pat. No. 5,476,496, it is known that in a bipolar pacing and sensing lead, the indifferent electrode or anode, typically in the form of a conductive ring disposed proximally of the tip electrode (which serves as the cathode), should have a large active surface area compared to that of the cathode. The objects of such an areal relationship are to reduce the current density in the region surrounding the anode so as to prevent needless or unwanted stimulation of body tissue around the anode when a stimulation pulse is generated between the cathode and anode, and to minimize creation of two focal pacing sites, one at the cathode and one at the anode which could promote arrhythmia. Typically, the total surface area of the anode is selected so as to be about two times to about six times that of the cathode.
4. Pacing Impedance
The design of a stimulation electrode typically carried at the distal tip of a body implantable lead must satisfy various requirements. An essential requirement is that a high impedance be provided at the tissue/electrode interface so as to decrease the current necessary for stimulation and consequently to increase the life span of the pulse generator battery without being electrically inefficient. A simple way to efficiently increase the interface impedance is to reduce the area of the active surface of the stimulation electrode. A relatively high impedance, for example, about 1,000 ohms, is a typical target value. (See, for example, U.S. Pat. No. 6,181,972.)
5. Left Side Stimulation and Sensing
The advantages of providing pacing therapies to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing in the respective heart chambers. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great vein, the left posterior ventricular (LPV) vein, or other coronary veins, proximate the left ventricle of the heart. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke. As used herein, the phrase “coronary sinus region” refers to the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other coronary vein accessible by way of the coronary sinus.
The tip electrode of a lead implanted in the coronary sinus region can pace and sense left side ventricular activity. When such a lead includes a second electrode proximal of the tip electrode and residing in the coronary sinus above the left ventricle closely adjacent to the left atrium of the heart, pacing and sensing of left atrial activity is made possible. Moreover, the lead may include one or more electrodes for the delivery of electrical shocks for terminating tachycardia and/or fibrillation. Such cardioverting/defibrillating electrodes may be used by themselves or may be combined with pacing and/or sensing electrodes.